1. Field of the Invention
The present invention relates generally to the atomic layer epitaxial (ALE) growth of thin films of semiconductors, but more particularly it relates to ALE growth of thin films of elemental semiconductors, i.e., silicon, germanium, tin, lead, and, in particular, diamond.
2. Description of the Prior Art
In conventional epitaxial growth systems such as molecular beam epitaxy, plasma-assisted epitaxy, photo-assisted epitaxy, or chemical vapor deposition, the deposited atoms seek their energetically most favorable position relative to one another and the substrate. For this to happen, the surface migration velocity of the deposited species must be sufficient to provide for the smooth and even, i.e., homogenous, growth of a single crystalline film. Energy is required to promote this surface migration velocity. This energy is obtained from the epitaxial process via substrate heat, plasma or photonic supplied energy, or exothermic chemical reactions. If adequate energy is not supplied, the growth will not be single crystalline. The absolute requirement for this energy needed to ensure an adequate surface migration velocity sufficient for a morphologically smooth surface and single crystalline growth limits the minimum temperature at which the film can be grown. As semiconductor device geometries are increasingly diminishing and dimensional tolerances are becoming a fraction of a micrometer, the resultant impurity diffusion created by high growth temperatures can no longer be an accepted mode of operation. Consequently, lower temperature techniques must be devised to nucleate and uniformly grow the semiconductors of interest. The atomic layer epitaxy (ALE) process has been shown to be efficacious in this respect for the processing of compound semiconductors, i.e., semiconductors which contain two or more elements from differing groups (columns) of the periodic table.
The atomic layer epitaxy (ALE) process, as far as could be determined, was first disclosed in U.S. Pat. No. 4,058,430 to Suntola et. al., filed on Nov. 25, 1975, and issued on Nov. 15, 1977. As disclosed in Suntola et. al., the ALE process was thought to be limited to compound semiconductors, but moreover, it was thought to be limited to compounds wherein the bonding energies between like cations and like anions were each less than that of the cation-anion bonding energy. More recently, however, it has been shown that other compounds, e.g., gallium arsenide (GaAs), are also amenable to ALE growth. The ALE process has been shown to have significant advantages in growing uniform layers of epitaxial thin semiconducting films of compound materials and in intrinsically excluding unwanted impurities from these films. As far as is known, however, the ALE process, with or without modifications, has never been used in the prior art to grow thin films of elemental semiconductors. A fortiori, the concept of using the ALE process for the growth of thin films of elemental semiconductors, at first glance, would seem inappropriate because in the traditional ALE process, the growth cyclically alternates between the growth of the cation and the anion species. Consequently, there is a need in the prior art to adapt the ALE process to the growth of thin films of elemental semiconductors, e.g., group IVB (where there are no well defined anion and cation species), while maintaining the traditional attributes of the ALE process.
Artifact diamond, an elemental semiconductor, has been sought for over 100 years. While artifact diamond crystals have been produced since the 1950's, thin films of diamond have only recently become available. Even so, thin films of diamond of semiconducting quality have not been obtained in sufficient quantities to be economically feasible, i.e., they are simply laboratory curiosities. The primary reason for this has been that the diamond surface reconstructs with double carbon, i.e., C.dbd.C or pi, bonds which leads to graphitic inclusions. Virtually all of the prior art methods of nucleating, synthesizing and growing diamond films have used high dilutions of atomic hydrogen (obtained from hot filaments or plasmas) to terminate the carbon bonds on the diamond surface to prevent the unwanted double pi bond reconstruction of the diamond surface. Unfortunately, atomic hydrogen bonds very tightly to the diamond surface and is not easily replaced by carbon from a hydrocarbon gas source. The result is inhomogeneity and multiple phase boundaries wherein the crystallites of diamond generally form at various orientations and in the morphology of abrasive sandpaper. Diamond synthesized in this manner also contains impurities from the hot filament or plasma chamber. Such material is not suitable for semiconductor purposes. Consequently, there is a need in the prior art to produce thin films o diamond of semiconducting quality by eliminating graphitic inclusions and process induced impurities, but yet in an economically viable fashion.
To continue, the presence of large quantities of hydrogen during the growth of diamond films has been thought to be absolutely necessary. Without it, diamond films exhibiting the definitive 1332 inverse centimeter RAMAN line have not been grown. Without large quantities of hydrogen, dangling C.dbd.C (pi) bonds have formed on the diamond surface. Hydrogen terminates the dangling carbon bonds and prevents the unwanted pi bonding. Hydrogen is also thought to be responsible for the absence of morphologically smooth single crystalline films. The reason for this is that the hydrogen binds to the diamond surface with a bonding strength greater than that of a diatomic C--C (sp.sup.3) bond as listed in the TABLE below. Accordingly, excited carbonaceous radicals compete with atomic hydrogen for the available carbon dangling bond sites on the growing diamond surface. Since the lowest free energy state is represented by the hydrogen termination of the diamond surface and not by termination with a carbonaceous radical or a carbon atom, it is indeed amazing that diamond grows at all using the prior art methods.
TABLE ______________________________________ CHEMICAL BOND STRENGTHS OF ELEMENTAL SEMICONDUCTORS RELATED TO ALE GROWTH Bond Description Bond Strength (Kcal/mol) ______________________________________ Br--Br 46.08 C--Br 95.6 C--N 174 C--O (diatomic) 256.7 C--O (as oxygen on diamond) 86 C.dbd.O 174 C--C (diatomic) 83 C--C (diamond) 144 C--H 99 C--F 117 (107) C--Cl 78 C--Si 104 Cl--Cl 57.87 C1--Pb 73 F--F 37.72 F--Si 126 F--C1 61.4 F--Sn 77 F--Pb 75 Ge--Ge 65 Ge--O 159 H--C 80.9 H--C1 103.1 H--F 135.8 H--H 104.18 H--I 71.4 H--Si 74.6 H--Ge 76.5 I--I 36.06 O--H 111 Pb--O 99 Si--O 192 Si--Si 76 Sn--O 133 Sn--Sn 46.7 ______________________________________
Currently, four different basic methods are used to grow diamond films in the prior art. The methods are (1) the hot filament method (with or without electric field bias), (2) the immersed plasma method, (3) the remote plasma method, and (4) the photo-assisted growth method. Variations and combinations of these methods are also used. All of the methods use carbonaceous gases highly diluted in hydrogen and, therefore, are costly (other methods using ion beams have not demonstrated definitive RAMAN lines). In all of the foregoing methods, only a fraction of the hydrogen molecules are decomposed into atomic hydrogen and these resultant hydrogen atoms are commonly thought to accomplish several functions. First, they extract hydrogen atoms already attached to the diamond surface by forming molecular hydrogen. This is seen in the TABLE above to be an energy efficient action as the H--H bond strength exceeds the C--H bond strength. Second, other hydrogen atoms attach to the nascent dangling carbon surface bonds. If this is not accomplished prior to the denudation of a second and adjacent carbon lattice site, the possibility exists for the formation of an unwanted C.dbd.C pi surface bond. Fortunately, there is a time constant associated with the formation of unwanted C.dbd.C bonds from the nascently formed adjacent dangling carbon surface bonds. This time constant is not known with great accuracy; however, it is thought to be generally sufficient to allow for the probability of attachment to the dangling surface bond of an sp.sup.3 -bonded methyl or an acetyl radical necessary for the continuation of diamond growth. Third, the atomic hydrogen is capable of severing a nascent C--CH.sub.3 surface bond to form a methane molecule and leave behind a dangling surface bond. This is the process of methanation which is evidenced by the much slower diamond growth rate as the carbonaceous reagent is increasingly diluted in molecular hydrogen. Thus, the probability that a denuded dangling carbon surface bond will be reterminated by a hydrogen atom is 10.sup.4 times more probable than its being reterminated by a carbonaceous gas radical. Fourth, the atomic hydrogen is believed to "etch" away any unwanted graphite that may result when unwanted C.dbd.C bonds are formed. It is thus seen that the hydrogen termination of the growing diamond surface prevents unwanted C.dbd.C bonds and graphite from forming, but that it is energetically favorable for the diamond surface to remain terminated by hydrogen rather than to grow. Hence, the probability of a continuous diamond film growth uninterrupted by hydrogen "inclusions" is energetically remote. A further complication of hydrogen in the growth of diamond is that hydrogen can be incorporated both interstitially (not on lattice sites) and substitutionally (as a replacement for carbon on a diamond lattice site). It is generally thought that the interstitial hydrogen is driven off at temperatures above 650 Celsius. Consequently, there is a need in the prior art to grow single crystalline diamond films in an improved manner such that hydrogen inclusion and, hence, grain boundaries are prevented.